Method of obtaining large grain size in silicon steel



Patented Dec. 12, 1950 METHQD 0F OBTAINING LARGE GRAIN SIZE IN SILECONSTEEL Everett W. Oppenheimer and Jack E. Lucas, Butler, 53a, assignorsto Armcc Steel Corporation, a corporation of Ohio No Drawing.Application February 20, 1947, Serial No. 729,876

8 Claims. i

Our invention has for its principal object the provision of an improvedprocedure for manufacturing silicon steel of good permeability and lowcore loss. It is applicable especially to the intermediate grades ofsilicon steel, which are produced under highly competitive conditions.

More specifically, our invention has as an object the attainment in aless expensive fashion of the combined features of low carbon contentand large grain size, and the attainment of improved magnetic qualitiesin the material as compared With the best commercially economicalprocedures hitherto known.

It has long been understood that a large grain size could be obtained bya critical strain procedure applied to silicon steel of finished gauge.This, however, involves subjecting the codreduced material to anintermediate, strainrelieving anneal, followed by a carefully controlledcritical strain in the nature of coldwork, preceding the final anneal.Such procedure is expensive, and the magnetic requirements forintermediate grades of silicon steel make it difficu t to accomplish.For low production costs it is necessary that the intermediate grades ofelectrical steel be produced by a routing in which the cold reducedstrip or sheet is given ony a continuous or open anneal after it reachesfinished gauge. One of the objects of the invention is to provide arouting which employs, after co d-reduction to gauge, a single heattreatment, but which nevertheless is productive of the improved results.

In a procedure outlined in Patent No. 2,358,788,

a relatively large grain size and low carbon is attained in the finalproduct by employing a phenomenon which may be termed grain growthinheritance, i. e. a large grain size is attained in the hot-roller,intermediate gauge product as a result of straining the product and thenbox annealing it with the hot mill scale still on its surfaces. As aresult of theattainment of this initial large grain size, after thesubsequent coldroling to gauge and fina open annealin the grain size isfound to be su stantially and usefully larger than would otherwise bethe case. The box annealing at around 1400 F. decarburizes the materialby causing the oxidation of carbon and the concurrent reduction of thehot mill scale; and the large grain size of the intermediate product isattained during this anneal.

But this procedure involves carefully controlled cold work on thehot-rolled intermediate product, which is a matter of inconvenience andexpense. Moreover, a type of scale which is quite diificult to pickleaway is left on the metal surfaces by the combination of the cold workon the scalded metal and the box anneal. Still other objects of thisinvention are the production of a large grain size in the final productwithout critical straining or other carefully controlled cold work atany stage of the routing, and an improvement of the scale conditionsresulting from a decarburizing anneal.

These and other objects of our invention which will be set forthhereinafter or will be apparent to one skilled in the art upon readingthese specifications, we accomplish by that procedure of which we willnow describe anexemplary' em-- bodiment.

In our invention. a simpler and less expensive method of producing theinitial large grain size has been found, and its influencepersists'through the cold reductions and shows up as an equally large orstill larger grain size than that hitherto obtainable after the finalopen anneal. This, in turn, results in improved magnetic properties.

In the practice of our invention, we start with a ferrous materialcontaining from about 1.00 to 3.50% silicon, depending on the magneticproperties required and subject it to the following routing:

1. The material is hot rolled to an intermediate gauge, say around .062inch.

2. The hot-rolled. product, with the hot mill scale still on itssurfaces, is box annealed at around 1200 to l700 (preferably 1400 F'.)..v

3. It is then open annealed at from 1700 to 2100 F., and pickled.

4. It is next cold-rolled to the desired finished gauge.

5. Finally .it is open annealed at 1700 to 2100 F.

The box anneal, which is step 2 of the routing, is at the propertemperature for decarburization, and in this anneal the carbon will bereduced to around .015 or lower.

Unlike the method of Patent 2,358,788, we do not attain the desiredlarge grain size in this anneal for the reason that we omit the previousstep of that patent in which the material was subjected to a carefullycontrolled amount of cold work.

We have discovered, however, that when the carbon has been reduced insilicon steel to around .015 or lower, an open anneal at around 2000 R,which is step 3 of our routing, causes the material to anneal rapidly toan unusually uniform and large grain size, although the material has notpreviously been strained by cold work.

The mechanism of this phenomenon is not fully explained. Apparently, therapid attainment of the large grain size is the result of the coactionof the decarburizing anneal and the high temperature open anneal, and isa phenomenon occurring with material of low carbon content.

By an open anneal we mean a continuous 01' strand anneal in which thematerial is passed in a single thickness through an elongated furnace,the surfaces of the material being open or exposed to the furnaceatmosphere, without limitation to the nature of the furnace atmosphere.

Since the material will be pickled immediately after the first openanneal (step 3) the nature of the furnace atmosphere during this step isunimportant. Ordinarily, we employ a furnace which is simply filled withthe products of combustion of the heating fuel. The temperature of theopen anneal should be varied with the silicon content for maximum grainsize. Thus, for materials varying from 1% to 3 70 silicon, thetemperature of this open anneal may be varied from about 1700 F. for thelower silicon value to about 2100 F. for the higher silicon value.

Another result of the combination of steps thus far commented upon isthat the scale remaining on the material after the initial open annealis a scale which is readily pickled away, much more so than is the scaleimmediately following the box anneal, and especially where there hasbeen previous cold work as in the process of Patent 2,358,788.

The final open anneal (step 5 of the process) is preferably carried onin a non-oxidizing or reducing atmosphere to avoid the necessity of asubsequent pickle. Here again the temperature of the anneal isadvantageously varied in accordance with the silicon content of thesteel as described above in connection with step 3.

When material treated in accordance with our process is pickled, coldreduced to the desired finished gauge, say .025-.014=", and finally openannealed, the resulting grain size will be found to be sufiicientlylarge to meet the requirements of the intermediate grades of siliconsteel. This again is believed to be a phenomenon of grain growthinheritance. But our materials have improved magnetic properties ascompared with products made by routings involving a small cold reductionahead of the box anneal.

Modifications may be made in our invention without departing from thespirit of it. Having thus described our invention in an exemplaryembodiment, what we claim as new and desire to secure by Letters Patentis:

1. A process of producing enhanced grain size in intermediate gradesilicon steel which comprises subjecting hot rolled silicon steel to abox anneal at substantially from 1200 to 1700 F. with the hot mill scalestill on its surfaces, to decarburize the steel to a carbon value nothigher than substantially .015%, and then without intervening coldreduction, subjecting the steel to an Open anneal at temperaturesbetween substantially 1700 and 2100 F., preparatory to producing furtherreduction in the thickness of the steel.

2. The process claimed in claim 1, in which the temperature of the saidopen anneal is varied directly with the silicon content of the steel.

3. The process of claim 2 followed by a cold rolling and a final openanneal at substantially 1700 to 2100 F.

4. The process of claim 3 in which the temperature is varied directlywith the silicon content.

5. A process of producing silicon steel of enhanced grain size and goodmagnetic properties which comprises subjecting hot rolled silicon steelat an intermediate gauge to a box anneal at substantially from 1200 to1700 F. with the hot mill scale still on its surfaces, to decarburizethe steel to a carbon content not substantially greater than ,015%,subjecting the decarburized steel without intervening cold work to anopen anneal at substantially 1700 to 2100 F. to produce a large grainsize therein, then cold rolling the steel to final gauge, and developingthe ultimate magnetic properties of the steel by an annealing treatment.

6. A process of producing silicon steel for magnetic uses which includesthe successive steps of subjecting hot rolled s'ilicon steel to a boxanneal at substantially 1200 to 1700 F. while the hot mill scale remainson the surfaces of the steel whereby to decarburize it to a carbon valuenot substantially greater than .015%, and then without interveningtreatment subjecting the steel to an open anneal at substantially 1700to 2100 F. whereby to develop a large grain size therein and to convertan remaining scale on the steel sur faces to a condition in which it canreadily be removed by pickling.

7. A process of producing silicon steel for magnetic uses whichcomprises hot rolling silicon steel containing substantially 1% to 3.5%silicon to an intermediate gauge, and without removing the hot millscale, box annealing the silicon steel at a temperature of substantially1200 to 1700 F., then without intervening treatment open annealing thesteel at a temperature of substantially 1700 to 2100 F., then coldrolling the steel to finished gauge, and developing its ultimatemagnetic properties by open annealing it at a temperature ofsubstantially 1700 to 2100 F.

8. The process of claim 7 in which the temperature of both open annealsis varied directly with the silicon'content of the steel.

EVERETT W. OPPENHEIMER. JACK E. LUCAS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,236,519 Carpenter Apr. 1, 19412,287,467 Carpenter et a1. June 23, 1942 OTHER REFERENCES Alloys of Ironand Silicon by Greiner,

McGraw Hill Book Co., New York, 1933, page 372.

5. A PROCESS OF PRODUCING SILICON STEEL OF ENHANCED GRAIN SIZE AND GOODMAGNETIC PROPERTIES WHICH COMPRISES SUBJECTING HOT ROLLED SILICON STEELAT AN INTERMEDIATE GAUGE TO A BOX ANNEAL AT SUBSCALE STILL ON ITSSURFACES, TO DECARBURIZE THE STEEL TO A CARBON CONTENT NOT SUBSTANTIALLYGREATER THAN .015%, SUBJECTING THE DECARBURIZED STEEL WITHOUTINTERVENING COLD WORK TO AN OPEN ANNEAL AT SUBSTANTIALLY 1700* TO2100*F. TO PRODUCE A LARGE GRAIN SIZE THEREIN, THEN COLD ROLLING THESTEEL TO FINAL GAUGE, AND DEVELOPING THE ULTIMATE MAGNETIC PROPERTIES OFTHE STEEL BY AN ANNEALING TREATMENT.